The present invention relates to a light source unit that uses light emitting elements such as semiconductor lasers for a plurality of different wavelength bands and is usable in an optical apparatus such as a projector, and to a projector.
For example, in an image display projector such as DLP (trademark) projector and a liquid crystal projector and in a photomask exposure apparatus, a high luminance discharge lamp (HID lamp) such as a xenon lamp and an ultra-high pressure mercury lamp has been used so far. As an example, a principle of a projector is described with reference to FIG. 7 that is a diagram for explaining a part of one kind of existing projectors (see Japanese Unexamined Patent Application Publication No. 2004-252112, etc.).
Light from a light source (SjA) that is formed of a high luminance discharge lamp or the like enters an incident end (PmiA) of a homogenizing means (FmA) with the help of focusing means (illustration thereof is omitted) that is formed of a concave reflector, lens, or the like, and entering light is output from an exit end (PmoA). Here, as the homogenizing means (FmA), for example, a light guide may be used. The light guide is also referred to as a rod integrator, a light tunnel, or the like, and is configured of a prism formed of a light transmissive material such as glass and a resin. The light entering the incident end (PmiA) is totally reflected repeatedly by side surfaces of the homogenizing means (FmA) and propagates through the homogenizing means (FmA), in accordance with the principle same as that of the optical fiber. Accordingly, even if distribution of light entering the incident end (PmiA) has unevenness, the homogenizing means (FmA) functions to sufficiently uniformize illuminance on the exit end (PmoA).
Note that, in addition to the light guide configured of a prism formed of a light transmissive material such as glass and a resin described above, there is a light guide that is a hollow square tube and whose inner surface is configured of a reflector. The light guide of this type performs the same function as that of the light guide configured of a prism, by propagating light while allowing the light to be reflected repeatedly by the inner surface.
An illumination lens (Ej1A) is disposed such that a square image of the exit end (PmoA) is formed on a two-dimensional light intensity modulator (DmjA). As a result, the two-dimensional light intensity modulator (DmjA) is illuminated with the light output from the exit end (PmoA). Incidentally, in FIG. 7, a mirror (MjA) is disposed between the illumination lens (Ej1A) and the two-dimensional light intensity modulator (DmjA). The two-dimensional light intensity modulator (DmjA) then so modulates the light as to direct the modulated light to a direction entering the image projection lens (Ej2A) or to a direction not entering the image projection lens (Ej2A) for each pixel, to display an image on a screen (Tj).
The two-dimensional light intensity modulator as described above is also referred to as a light valve. In the case of the optical system of FIG. 7, normally, DMD (trademark, digital micro-mirror device) is often used as the two-dimensional light intensity modulator (DmjA).
In addition to the above-described light guide, the homogenizing means includes a fly eye integrator. A principle of a projector using the fly eye integrator as the homogenizing means is described with reference to FIG. 8, as an example. FIG. 8 is a diagram for explaining a part of one kind of existing projectors (see Japanese Unexamined Patent Application Publication No. 2001-142141, etc.).
Light from a light source (SjB) configured of a high luminance discharge lamp or the like enters, as substantially parallel luminous flux, an incident end (PmiB) of homogenizing means (FmB) configured of a fly eye integrator with the help of collimator means (illustration thereof is omitted) formed of a concave reflector, lens, or the like, and resultant light is output from an exit end (PmoB). Here, the homogenizing means (FmB) is configured of a combination of a front fly eye lens (F1B) on incident side, and a rear fly eye lens (F2B) and an illumination lens (Ej1B) on exit side. Each of the front fly eye lens (F1B) and the rear fly eye lens (F2B) is formed by arranging a plurality of square lenses that has the same focusing distance and the same shape, vertically and horizontally.
Each of the front fly eye lenses (F1B) and the rear fly eye lens (F2B) corresponding thereto configure Kohler illumination optical system. A plurality of Kohler illumination optical systems is thus arranged vertically and horizontally. Typically, Kohler illumination optical system is configured of two lenses, and a front lens collects light to illuminate a target surface (a surface desired to be illuminated) uniformly. At this time, the two lenses are disposed so that the front lens forms a light source image not on the target surface but on a center of a surface of a rear lens and the rear lens forms an image of an outer square shape of the front lens on the target surface. The action of the rear lens is to prevent phenomenon in which illuminance in the periphery of the square image formed on the target surface is dropped depending on the size of the light source when the light source is not a complete point light source but has a finite size. The phenomenon occurs when the rear lens is not provided. It is possible to uniform illuminance over the periphery of the square image on the target surface by the action of the rear lens, without depending on the size of the light source.
Here, in the case of the optical system in FIG. 8, substantially parallel luminous flux basically enters the homogenizing means (FmB). Therefore, the front fly eye lens (F1B) and the rear fly eye lens (F2B) are disposed such that a distance therebetween becomes equal to the focusing distance thereof, and therefore, an image on the target surface of uniform illumination as Kohler illumination optical system is generated to the infinity. Incidentally, since the illumination lens (Ej1B) is disposed on a rear stage of the rear fly eye lens (F2B), the target surface is drawn on a focusing surface of the illumination lens (Ej1B) from the infinity. Each of the plurality of Kohler illumination optical systems arranged vertically and horizontally is parallel to an incident optical axis (ZiB), and luminous flux enters each of the Kohler illumination optical systems substantially axisymmetrically to the center axis thereof. Therefore, output luminous flux is also axisymmetrical. Accordingly, images of the outputs of all of the Kohler illumination optical systems are formed on the same target surface on the focusing surface of the illumination lens (Ej1B) by property of the lens in which light beams entering a lens surface at the same angle are so refracted as to travel toward the same point on the focusing surface irrespective of incident positions of the respective light beams on the lens surface, namely, by Fourier transform function of the lens.
As a result, illumination distributions on the respective lens surfaces of the front fly eye lenses (F1B) are all overlapped, and thus a synthesized square image whose illuminance distribution is more uniform than that in the case of one Kohler illumination optical system, is formed on the incident optical axis (ZiB). When the two-dimensional light intensity modulator (DmjB) is disposed on the position of the synthesized square image, the two-dimensional light intensity modulator (DmjB) that is an illumination target is illuminated with the light output from the exit end (PmoB). In the illumination, a polarization beam splitter (MjB) is disposed between the illumination lens (Ej1B) and the two-dimensional light intensity modulator (DmjB) to reflect the light toward the two-dimensional light intensity modulator (DmjB). The two-dimensional light intensity modulator (DmjB) modulates the light and reflects the modulated light such that the polarization direction of light for each pixel is rotated by 90 degrees or is not rotated, according to a picture signal. As a result, only the rotated light passes through the polarization beam splitter (MjB) and enters an image projection lens (Ej3B), thereby displaying an image on the screen (Tj).
In the case of the optical system in FIG. 8, typically, LCOS (trademark, silicon liquid crystal device) is often used as the two-dimensional light intensity modulator (DmjB). In a case of such a liquid crystal device, only a component of light in a specified polarization direction is effectively modulated. Therefore, a component of light parallel to the specified polarization direction is normally transmitted as is. However, in the optical system in FIG. 8, a polarization aligning device (PcB) that rotates polarization direction of only a component of light perpendicular to the specified polarization direction by 90 degrees and consequently allows all of light to be effectively used may be interposed, for example, on a rear stage of the rear fly eye lens (F2B). In addition, for example, a field lens (Ej2B) may be interposed immediately before the two-dimensional light intensity modulator (DmjB) so that substantially parallel light enters the two-dimensional light intensity modulator (DmjB).
Incidentally, in addition to the reflective two-dimensional light intensity modulator illustrated in FIG. 8, a transmissive liquid crystal device (LCD) is also used with a compatible optical arrangement as the two-dimensional light intensity modulator (see Japanese Unexamined Patent Application Publication No. H10-133303, etc.).
Incidentally, in a common projector, to perform color display of an image, for example, a dynamic color filter such as a color wheel is disposed on the rear stage of the homogenizing means to illuminate the two-dimensional light intensity modulator with color sequential luminous fluxes of R (red), G (green), and B (blue), and color display is thus achieved time-divisionally. Alternatively, an optical system in which a dichroic mirror or a dichroic prism is disposed on the rear stage of the homogenizing means to illuminate the two-dimensional light intensity modulator that is provided independently for each color, with light color-separated to three primary colors R, G, and B, and a dichroic mirror or a dichroic prism is disposed to perform color synthesis of the modulated luminous fluxes of the three primary colors R, G, and B is configured. However, to avoid complication, these are omitted in FIG. 7 and FIG. 8.
However, the high luminance discharge lamp disadvantageously has low conversion efficiency from supplied power to optical power, namely, large heating loss, short lifetime, or the like. As an alternate light source overcoming these disadvantages, a solid-state light source such as an LED and a semiconductor laser has attracted attention in recent years. Among them, the LED has smaller heating loss and longer life time as compared with the discharge lamp. However, light radiated from the LED does not have directivity similarly to the discharge lamp. Thus, usage efficiency of light is disadvantageously low in an application using only light in a certain direction, such as the projector and an exposure apparatus.
On the other hand, the semiconductor laser has a disadvantage that speckle occurs due to high coherency, but the disadvantage is overcome by various technical improvement such as usage of a diffuser plate. Since the semiconductor laser has small heating loss and long lifetime similarly to LED and has high directivity, the semiconductor laser advantageously has high usage efficiency of light in application using only light in a certain direction, such as the projector and the exposure apparatus described above. Moreover, the semiconductor laser utilized high directivity to perform optical transmission by optical fibers with high efficiency. Therefore, it is possible to separate the installation position of the semiconductor laser from the position of a projector or the like using the light. Consequently, it is possible to enhance flexibility of device designing.
Incidentally, even in the case where the same current flows, emission wavelength and light emitting intensity of the semiconductor laser vary due to environment temperature variation or temperature increase by self heating, and due to deterioration associated with the increase of accumulated conduction time. In the case where the semiconductor laser is used for a part or all of the three primary colors R, G, and B as a light source of the projector, color and brightness of the entire image vary due to such variation. Therefore, in the case where the semiconductor laser is applied to a high-fidelity projector, it is necessary to perform stabilization of color, namely, stabilization of white balance and stabilization of brightness.
When white light is fabricated by mixing light from light sources of three primary colors R, G, and B, mixing ratio of the three primary colors may be normally adjusted so that correct white light is obtained, while measuring chromaticity with use of a color meter when a human performs the fabrication manually. On the other hand, in a projector, it is difficult to achieve automatic adjustment with low cost. The above-described color meter is expensive and is not easily incorporated in a projector. Therefore, it is forced to use an inexpensive optical sensor suitable for incorporation. Even if an inexpensive optical sensor is only used, however, a precise spectral filter with high cost is necessary to obtain a function equivalent to that of the color meter. Therefore it is necessary to achieve a configuration in which the spectral filter is in place with an inexpensive filter having simple specification. However, technology in which a quantity correlated with chromaticity is measured with use of an inexpensive optical sensor or filter and supplied power to the semiconductor laser of R, G, and B is efficiently adjusted automatically based on the measurement result has not been established so far.
Technology to avoid disadvantage of phenomenon in which emission wavelength is particularly varied in a case where a semiconductor laser or LED is applied as a light source has been developed. For example, in Japanese Unexamined Patent Application Publication No. 2006-252777, there is disclosed a technology in which it is determined whether gradient of the spectral sensitivity characteristics is varied in a direction with long emission wavelength or in a direction with short emission wavelength, or is not varied, based on light intensity detection that is performed for emission wavelength band of a light source with use of a positive optical sensor and a negative optical sensor, and a reference level of power supply control of light sources of colors R, G, and B is increased or decreased based on the determination result.
In the case of this technology, however, only the direction of the temporal variation of the emission wavelength is detected and controlled. Therefore, color variation at relatively high rate associated with temperature variation caused by heat generation of the light source itself immediately after turning on of the light source is corrected, but the color variation associated with moderate variation of the environment temperature and deterioration of the light source over long term is not corrected disadvantageously. Moreover, power supply control for each of the color light sources in the case where color variation of the light sources of a plurality of colors occurs independently has not been solved.
Further, for example, in Japanese Unexamined Patent Application Publication No. 2007-156211, there is disclosed a technology allowing light sources of the respective colors R, G, and B to emit light color-sequentially. By the technology, white balance is corrected by performing control such that difference between outputs of the optical sensors and the target values thereof becomes small while assuming that the spectral sensitivity distribution of the optical sensors of the respective colors R, G, and B is equivalent to that of the color matching function in the XYZ color system recommended by Commission International de I'Éclairage (CIE).
In the feedback control of the white balance, however, variation of the supplied power of each of the light sources of the three colors to focus the output of the optical sensors to the target values has not been solved.
Moreover, in Japanese Unexamined Patent Application Publication No.
2008-134378, there is disclosed a technology in which an angle of a dichroic mirror is varied based on a detection result of an photodetection sensor that detects output and a color from an LED light source, and undesirable wavelength component of light emitted from the LED is discarded to correct color. However, the technology has low efficiency due to the discard of undesirable light, and a method of achieving a photodetection sensor detecting color is not developed.